Democratic Sen. Ben Nelson of Nebraska announced Tuesday that he will not run for re-election next year .

Nelson , 70 , said in a statement that it was time to `` step away from elective office , spend more time with my family , and look for ways to serve our state and nation . ''

`` Simply put : It is time to move on , '' Nelson said in the statement .

Despite his record as a conservative Democrat who sometimes voted against his party , Nelson had been considered a top target of Republicans heading into the 2012 elections .

A former two-term governor of Nebraska , Nelson narrowly won election to the Senate in 2000 and easily won re-election in 2006 .

His departure increases the difficulty for Democrats to retain their Senate majority next year .

Democrats currently have a 53-47 margin in the Senate , including two independents who caucus with them . In the 2012 elections , 23 of the 33 seats up for grabs are currently held by Democrats and the two independents .

So far , six other Democratic senators and two Republicans have announced they will retire instead of running for re-election next year .

A Senate Democratic leadership aide , speaking on condition of not being identified , told CNN that party officials are reaching out to former Nebraska Sen. Bob Kerrey to possibly run for Nelson 's seat . Kerrey , 68 , is a Medal of Honor winner from the Vietnam War who , like Nelson , served as both a Democratic governor and senator .

Nelson was known for looking out for his state 's interests , especially on agriculture issues . He sided with Republicans on some high-profile issues , opposing the Supreme Court nomination of Elena Kagan and a climate-change measure pushed by the White House and Democrats .

President Barack Obama commended Nelson for working with both parties during his career , saying in a statement it was `` a trait far too often overlooked in today 's politics . ''

Jim Manley , the former long-time spokesman for Senate Majority Leader Harry Reid , D-Nevada , said Nelson was often a `` thorn in Harry Reid 's side . '' However , Manley said Reid `` always respected the fact that Nelson was trying to do what was best for Nebraska . ''

According to the Senate Democratic leadership aide , Reid and Sen. Chuck Schumer , D-New York , tried to convince Nelson to stay in Congress after Nelson told a local newspaper a few weeks ago he was still deciding on whether to run .

`` Over the last several weeks , there were intense conversations with Reid and Schumer trying to make Nelson understand how needed he was and how valued he is as a member of the caucus , '' the Senate aide said . `` Ultimately they understood that this is a personal decision . ''

A Democratic strategist who spoke on condition of not being identified characterized Nelson 's decision as disloyal to the party .

`` He typically takes the easy way out , '' the strategist said . `` The party has stood by him , but when the going gets tough , he abandons the party . ''

In his statement , Nelson called for `` those who will follow in my footsteps to look for common ground and to work together in bipartisan ways to do what 's best for the country , not just one political party . ''

`` Public office is a place for public service , not personal profit , '' Nelson 's statement said . `` It 's about promoting the common good , not the agenda of the radical right or the radical left . It 's about fairness for all , not privileges for the few . And , it 's about protecting the rights of individuals , even if it angers the majority . I hope and believe I have fulfilled these principles to the best of my ability . ''

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Party officials are reaching out to former Sen. Bob Kerrey regarding Nelson 's seat

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Democratic Sen. Ben Nelson announces he will not seek re-election

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Nelson was expected to face a tough Republican challenge next year

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The former Nebraska governor is a conservative Democrat